Method of making hole in ceramic green sheet

ABSTRACT

A ceramic green sheet is irradiated with laser beam of high-peak short-pulse type to form a hole in the sheet. In the laser beam, the minimum oscillation output more than 60% of the maximum oscillation output. This prevents material of the ceramic green sheet from melting partially and remaining around the hole. Therefore, a multi-layer assembly of the ceramic green sheets built by joining the sheets in layers and baking the joined sheets hardly has a structural defect around a via-electrode provided in the hole.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of making a hole in a ceramic green sheet used for manufacturing a ceramic device, such as a ceramic multi-layer device.

BACKGROUND OF THE INVENTION

[0002] A ceramic multi-layer device, such as a small filter or a module component, is often manufactured by stacking ceramic green sheets one over another. In the manufacturing, the ceramic green sheets are provided at their surfaces with patterns of wiring by screen printing the patterns, assembling the sheets in layers, baking the sheets, and dividing the sheets into pieces of ceramic multi-layer devices. The ceramic green sheets are ceramic layers to construct the ceramic multi-layer device.

[0003] The patterns of wiring provided on both sides of the ceramic layer are connected to each other by a via-electrode extending across the ceramic layer. The via-electrode is fabricated by filling a via-hole in the ceramic green sheet with paste of electrically conductive material and then by baking the paste. The ceramic green sheets may be doped with additive, such as glass, for baking the sheets at a low temperature.

[0004] As required to have a small overall size and an improved performance, the ceramic multi-layer devices are demanded to have larger number of the via-holes made in the ceramic green sheets having small diameters in a shorter time.

[0005] The via-holes are made by punching the ceramic green sheets with a die. The smaller the diameters of the via holes, the shorter the operating life of the die and pins of the die is. The punching does not provide via-holes having diameters smaller than 100 μmΦ easily and provides via-holes at a small processing speed.

[0006] As disclosed in Japanese Patent Laid-Open Publication No. 01-9691, via-holes may favorably be made by laser beam of CO₂, YAG, or excimer. This method can provide via-holes having diameters smaller than 100 μmΦ easily while not requiring particular maintenance operations, such as a change of a die.

[0007] The laser beam can make the via-holes fast by repetitively irradiating the ceramic green sheet with the laser beam with a galvano-scan mirror having a reflecting angle controlled by electronic control without moving of the ceramic green sheets. This allows the via-holes to be provided at a speed substantially 100 times larger than that of the punching.

[0008]FIG. 8 is a cross sectional view of a baked ceramic assembly in a conventional method. FIG. 9 illustrates a conventional via-hole.

[0009] In the technique using the laser beam, the ceramic green sheet containing particularly glass material may include a melting portion of the glass material while being irradiated with the laser beam to provide the via holes. The melting portion of the glass material appears as a melting substance 17 remaining at an edge of via-hole 16, as shown in FIG. 9.

[0010] The melting substance 17 prevents the via-hole 16 from being filled with the paste of electrically conductive material at the succeeding process for fabricating via-electrode 12. Portions around the via-hole 16 in the ceramic green sheet may deteriorate by heat of the laser beam, hence significantly declining a joining tightness and wet property between the electrically conductive material paste and the ceramic green sheet.

[0011] The baked ceramic assembly 10 is provided by forming the via-holes 16 in the ceramic green sheets by the laser beam, filling the via-holes 16 with the electrically conductive material paste, assembling the sheets in layers, and baking the sheets. As shown in FIG. 8, the baked ceramic assembly 10 may have structural defect 13 as a cavity around via-electrode 12. The defect reduces connecting property as well as the resistance to moisture, thus reducing long-term operational reliability of the ceramic multi-layer device.

SUMMARY OF THE INVENTION

[0012] A surface of a ceramic green sheet is irradiated with laser beam of a substantially square waveform pulse in which a minimum of an output is 60% of a maximum of the output to provide a through-hole in the ceramic green sheet. This method prevents no portion of material of the ceramic green sheet from melting and burred around the hole.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 illustrates a method of making a hole in a ceramic green sheet according to Exemplary Embodiment 1 of the present invention.

[0014]FIG. 2 illustrates a waveform of a high-peak short-pulse laser beam used in the method of Embodiment 1.

[0015]FIG. 3 illustrates a waveform of a high-peak short-pulse laser beam used in the method of Embodiment 1.

[0016]FIG. 4 is a cross sectional view of a baked ceramic assembly in the method of Embodiment 1.

[0017]FIG. 5 illustrates a method of making a hole in a ceramic green sheet according to Exemplary Embodiment 2 of the invention.

[0018]FIG. 6 illustrates the method of making a hole in a ceramic green sheet according to Embodiment 2.

[0019]FIG. 7 illustrates a waveform of a pulse laser beam in a conventional method.

[0020]FIG. 8 is a cross sectional view of a baked ceramic assembly.

[0021]FIG. 9 illustrates a conventional via-hole formed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Exemplary Embodiment 1

[0022]FIG. 1 illustrates a method of making a hole in a ceramic green sheet according to Exemplary Embodiment 1 of the present invention. FIG. 2 illustrates the waveform of a high-peak short-pulse laser beam used in the method. FIG. 4 is a cross sectional view of a baked ceramic assembly in the method of Embodiment 1.

[0023] Ceramic slurry is prepared by mixing non-organic combination of ceramic powder mainly containing Al₂O₃ and silicate glass including alkali earth metal oxide with organic binder, plasticizer, and solvent. The ceramic slurry is then applied over a carrier film with a doctor blade, thus providing a wet ceramic green sheet. The wet ceramic green sheet is then continuously dried to provide a ceramic green sheet 3. The carrier film may be, but not limited to, made of polyethylene terephthalate (PET).

[0024] Then, the ceramic green sheet 3 is placed on an X-Y table 4 of a laser processing machine and irradiated with laser beam 2 having a square pulse waveform and focused by a condenser lens 1, thus providing a through-hole.

[0025] The laser beam 2 is a high-peak short-pulse laser beam having a square pulse waveform 7 shown in FIG. 2.

[0026]FIG. 2 illustrates an ideal shape of the pulse waveform 7. The pulse waveform 7 shown in FIG. 3 shows an actual shape of the high-peak short-pulse laser beam 2 reaching the ceramic green sheet 3. The pulse waveform 7 particularly in which the minimum power 9 is not smaller than 60% of the maximum power 8 provides a hole having a preferable shape, an easy control of conditions for the irradiating, and a small amount of a remaining substance around the hole.

[0027]FIG. 7 illustrates a pulse waveform 14 of CO₂ laser beam in a conventional method. Since having a small peak output of 500 W, the CO₂ laser beam needs to have a long pulse width for providing the hole. The pulse waveform 14 in the conventional method has a long rising time. It takes some tens microseconds until the output of the laser beam becomes zero after turning off the laser beam, hence creating a domain 15 where the oscillation output is not controlled. The longer the pulse width, the greater the domain 15 becomes. Accordingly, the laser beam in the conventional method does not control the oscillation output and the pulse width easily and hardly be determined to desired settings for process.

[0028] Since having the small peak output, the laser beam in the conventional method has the pulse width increased to provide a desired shape of the hole and provides a thermal influence to the edge of the hole 16 in the ceramic green sheet 3.

[0029] As the result, the ceramic green sheet 3 containing glass material is not sufficiently heated up at the edge of the hole 16, and a melting portion 17 may remain around the hole 16 when being cooled down. If a baked ceramic assembly 10 is fabricated from the ceramic green sheets 3 having such unfavorable hole, the assembly 10 may includes a structural defect 13 around via-electrode 12, as shown in FIG. 8.

[0030] In order to prevent the melting portion 17 shown in FIG. 9 from remaining around the hole 16, the peak output of the high-peak short-pulse laser beam is preferably more than 700 W. A duration from that oscillation of the laser beam is turned off to that the output becomes zero is preferably not longer than 2 μs. A pulse width 18 may preferably be controlled to be shorter than 2 μs.

[0031] The amount of the melting substance 17 shown in FIG. 9 becomes small if a cycle of heating and cooling is not repeated many times. The ceramic green sheet 3 may be preferably irradiated with the laser beam 2 not more than three times.

[0032] The ceramic green sheet 3 is provided with the through hole 16 therein, and then, the paste of electrically conductive material is applied to fill the hole 16, thus providing the via-electrode 12.

[0033] Then, patterns 11 of wiring are printed on the ceramic green sheets 3. The ceramic green sheets 3 having the wiring patterns 11 thereon are stacked one on another and joined together by a thermal bonding technique to provide a ceramic green sheet block.

[0034] The ceramic green sheet block is then degreased at a temperature ranging from 350 to 600° C. and baked at a temperature ranging from 850 to 950° C. to provide a baked ceramic assembly 10 shown in FIG. 4.

[0035] The baked ceramic assembly 10 is provided with a SAW filter and other chip components mounted thereon by a surface mounting technique, and is then divided by dicing or brake-up into desired sizes of multi-layer ceramic devices.

[0036] According to Embodiment 1, the material of the ceramic green sheet 3 is not limited to combination of ceramic powder essentially containing Al₂O₃ and silicate glass including alkali earth metal oxide. The method of Embodiment 1 is specifically effective to making the through-hole in a sheet of low-temperature baking ceramic material which can be baked at a temperature lower than 1000° C. since the material often provides the melting substance by laser beam.

Exemplary Embodiment 2

[0037]FIGS. 5 and 6 are schematic views showing a method for making a hole in a ceramic green sheet 3 according to Exemplary Embodiment 2 of the invention. Preparing of the ceramic green sheets 3 and fabricating of a multi-layer ceramic device are identical to those of Embodiment 1 and will be described in no more detail.

[0038] According to Embodiment 2, the ceramic green sheet 3 is supported by a carrier film 5 and placed on an X-Y table 4. A side of the ceramic green sheet 3 opposite to the carrier film 5 is irradiated with laser beam 2 shown in FIGS. 2 and 3 for making a hole 16 in the sheet.

[0039] This method allows the ceramic green sheet 3, even if being thin, to be supported by the carrier film 5, thus making holes in the sheet continuously and increasing productivity.

[0040] When the ceramic green sheet 3 is irradiated particularly directly with the laser beam 2, the surface of the sheet may be polluted with a melting substance.

[0041] For protecting the surface, the ceramic green sheet 3 may preferably be coated with a resin film 6, such as an adhesive coated PET film, as shown in FIG. 6.

[0042] The resin film 6 may not be coated with an adhesive layer if being attached securely to the surface of the ceramic green sheet 3. The material of the resin film 9 is not limited to PET.

[0043] Table 1 illustrates the relationship between the energy of the laser beam 2 of a high-peak short-pulse laser and a status of the hole made by the laser beam 2. TABLE 1 Energy 1 2 5 10 20 30 50 60 (mJ) Removed 4 5 12 25 50 70 120 Not Volume Measur- (×10⁵ mm³) able Status (*1) No No No No No No (*2) Prob- Prob- Prob- Prob- Prob- Prob- lem lem lem lem lem lem

[0044] As shown in Table 1, an excessively-large power of the energy provides a breaking of the ceramic green sheet 3, fusion of the ceramic green sheet 3 and the carrier film 5, and removal of the ceramic green sheet 3 from the carrier film 5. This may cause a short-circuiting during the application of electrically conductive paste in a later step, prevents the carrier film 5 from removing properly from the green sheet 3 in a later step.

[0045] When the energy of the laser beam 2 is too small, the edge at both, upper and lower, openings of the hole 16 is tapered. Accordingly, the hole 16 is hardly formed to have a desired shape, and has more amount of the melting substance 17.

[0046] The energy of the high-peak short-pulse laser beam 2 preferably ranges from 2 to 50 mJ from Table 1.

[0047] If the ceramic green sheet 3 is irradiated through the carrier film 5 with the laser beam 2, the diameter of the resultant hole may have a diameter at a surface facing the side opposite to the carrier film 5 smaller than a diameter at the surface facing the film 5. In this case, the electrically conductive paste filled in the hole for forming the via-electrode 12 is attached to the carrier film 5 and removed from the ceramic green sheet 3 together with the carrier film 5 when the film 5 is detached from the ceramic green sheet 3. If the ceramic green sheet 3 is thin, the surface of the sheet opposite to the carrier film 5 is preferably irradiated with the laser beam 2.

[0048] As set forth above, the methods of making holes according to Embodiments 1 and 2 reduces the amount of the melting substance around the hole generated by the laser beam. The method reduces the structural defect near the via-electrode, thus assuring operational reliability of the resultant multi-layer ceramic device. 

What is claimed is:
 1. A method of making a hole in a ceramic green sheet, comprising the steps of: preparing a ceramic green sheet; and irradiating a first surface of the ceramic green sheet with laser beam having a substantially square pulse shape having a minimum power is more than 60% of a maximum power.
 2. The method according to claim 1, wherein an oscillation output of the laser beam is not smaller than 700 W.
 3. The method according to claim 1, wherein a time for which oscillation of the laser beam can be controlled is not longer than 2 μs.
 4. The method according to claim 1, wherein said step of irradiating the first surface of the ceramic green sheet with the laser beam comprises irradiating the first surface of the ceramic green sheet with the laser beam not more than three times for making the hole.
 5. The method according to claim 1, wherein an energy of the laser beam ranges from 2 to 50 mJ.
 6. The method according to claim 1, wherein said step of preparing the ceramic green sheet comprises preparing the ceramic green sheet having a second surface thereof coated with a carrier film.
 7. The method according to claim 1, wherein said step of preparing the ceramic green sheet comprises preparing the ceramic green sheet havin the first side coated with a protective film. 